1. Field of the Invention
This invention relates to a multi-layered printed wiring board having a plurality of wiring layers, and more particularly to a multi-layered printed wiring board that is suitably applied to achieve high-density wiring.
2. Description of the Related Art
In recent years, with the advancement of functions of digital equipment, an increasing number of printed wiring boards installed on digital equipment have been equipped with a great number of multi-pin packaged semiconductor devices such as the BGA (Ball Grid Array). Such multi-pin packaged semiconductor devices include integrated circuits having 500 pins or more. The wiring density of such printed wiring boards has been raised with the increase in the number of pins. This necessitates increasing the number of wiring layers of printed wiring boards, and if a multi-layered printed wiring board having four layers cannot house all required lines, a multi-layered printed wiring board having six layers or a multi-layered printed wiring board having eight layers is used.
Japanese Laid-open Patent Publication (Kokai) No. 10-270862 discloses an example of conventional multi-layered printed wiring boards (first prior art). According to this prior art, a plurality of circuit elements are mounted on the multi-layered printed wiring board, and a ground layer, a signal layer, and a power supply layer providing the circuit elements with power supply voltages are-laminated one upon another via respective insulating materials. The respective circuit elements are classified into a plurality of groups according to the operating speed, and regions for mounting the circuit elements on the multi-layered printed wiring board are determined according to the groups. In the power supply layer, power supply lines are formed according to the groups, and a power line, which separates the power supply lines from one another against high frequencies, connects between the power supply lines belonging to different groups. In this connection, the multi-layered printed wiring board must be provided with a ground-only layer comprised only of ground lines and a power supply-only layer comprised only of power supply lines in order to reduce radiation noises.
Further, Japanese Laid-open Patent Publication (Kokai) No. 7-321429 discloses an example of conventional double-layered printed wiring boards (second prior art). In the printed wiring board according to this prior art, ground lines and power supply lines are provided in the form of a grid in each of the wiring layers such that they extend in parallel and are alternately arranged, and the wiring layers are arranged such that the lines of a first wiring layer and those of a second layer vertically intersect each other, and the ground lines and the power supply lines in the respective wiring layers are connected together via through holes. This arrangement enables a solid ground and reduces radiation noises.
The above described prior arts, however, have the following problems:
The increase in the number of layers as in the first prior art to accommodate all required lines easily realizes high-density wiring, but increases the time and cost required for manufacturing the printed wiring board.
Providing ground lines and power supply lines in the form of a grid on each of the wiring layers as in the second prior art reduces radiation noises, but imposes a lot of restrictions on a space for wiring signal lines and makes it difficult to achieve high-density wiring.